1. Field of the Invention
The present invention relates to an electrophotographic color image forming apparatus capable of protecting images from degradation ascribable to residual toner and reversely transferred toner present on a photoconductive element and reducing blur ascribable to nitric acid and metal nitrates derived from discharge products and depositing on the photoconductive element.
2. Description of the Background Art
Today, a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus provided with a cleanerless configuration, i.e., not including a cleaning unit is attracting attention from the size and cost reduction standpoint. Japanese Patent Laid-Open Publication Nos. 10-240004 and 2000-321849, for example, each propose a cleanerless image forming apparatus in which a developing unit feeds toner to a photoconductive element for forming a toner image and, at the same time, collects residual toner left on the photoconductive element after image transfer. On the other hand, Japanese Patent Laid-Open Publication No. 11-282320, for example, discloses a cleanerless image forming apparatus in which a brush, held in contact with a photoconductive element, temporarily collects residual toner and then returns it to the photoconductive element at preselected timing, so that the toner is collected in a developing unit. The above document teaches that the function of the brush may be assigned to a charger.
The problem with an electrophotographic image forming apparatus is that discharge from a charger, which uniformly charges the surface of a photoconductive element, produces NOx (nitrogen oxides), ozone and other discharge products. NOx, for example, reacts with moisture present in air to thereby produce nitric acid that, in turn, reacts with metal to produce metal nitrates. Forming a thin film on the photoconductive element, such nitric acid and metal oxides absorb moisture and thereby lower the surface resistance of the photoconductive element. As a result, a latent image formed on the photoconductive element is disturbed with the result that the resulting toner image is blurred.
While an image forming apparatus of the type including a cleaning unit can shave off the film of nitric acid and metal nitrates with a cleaning blade, the cleanerless image forming apparatus cannot do so. In light of this, the cleanerless image forming apparatus taught in Laid-Open Publication No. 10-240004 mentioned earlier uses a developing roller or an image transfer roller as grinding means for shaving off the above film.
The cleanerless image forming apparatus disclosed in Laid-Open Publication No. 2000-321849 also mentioned earlier teaches that a charger accommodates an optical catalyst therein for decomposing the discharge products.
However, none of the conventional cleanerless image forming apparatuses is provided with an implementation for reducing the amount of residual toner to remain on the photoconductive element. Consequently, in the apparatus of the type effecting development and cleaning at the same time, irregular charging and defective exposure become more likely to occur with an increase in the amount of residual toner, lowering image quality little by little. In the apparatus of the type using a brush, when much residual toner is present or in a repeat print mode, it is likely that residual toner deposits on the brush in an amount too much to be held by the brush, aggravating irregular charging and defective exposure and lowering image quality little by little. In the repeat print mode, if the residual toner deposited on the brush is returned to the photoconductive element and then collected in the developing unit every time a preselected number of prints are output, the ability of the brush and therefore image quality may be enhanced. This scheme, however, lowers printing speed.
A color image forming apparatus has a problem that when toner images of different colors are sequentially transferred from photoconductive elements to a recording medium, intermediate image transfer body or similar subject body, a toner image already transferred to the subject body is reversely transferred to the photoconductive drums. In the cleanerless system, the toner reversely transferred brings about irregular charging and defective exposure in combination with the residual toner, lowering the image quality of the next color. Moreover, because the reverse transfer of toner occurs in the preceding step, the toner collected in the developing unit in the following step gives rise to a color mixture problem, further lowering image quality.
In a color image forming apparatus of the type developing latent images formed on a single photoconductive element with toner of different colors, the residual toner is also derived from the preceding step and therefore aggravates color mixture in the individual developing unit.
The residual toner and reverse toner transfer are ascribable mainly to the fact that the amount of charge differs from one toner grain to another toner grain. Further, the residual toner and reversely transferred toner include even toner grains charged to polarity opposite to expected polarity and unable to move in a preselected image transfer direction.
When grinding means is used to shave off the thin film of nitric acid and metal nitrates, as taught in Laid-Open Publication No. 10-240004, the grinding means shaves off the surface of the photoconductive element at the same time, causing the photoconductive element to wear little by little.
The optical catalyst scheme disclosed in Laid-Open Publication 2000-321849 has a problem that the optical catalyst must be positioned in close proximity to the surface of the photoconductive element and charger, i.e., needs an exclusive space in the vicinity of the photoconductive element and charger. Positioning the optical catalyst close to the photoconductive element and charger is contradictory to size and cost reduction achievable with the cleanerless system.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-301977, 9-50214, 10-340030 and 2000-98855.